Solid free-form fabrication (SFF) is a designation for a group of processes that produce three dimensional shapes from additive formation steps. Most SFF processes are also referred to as layer additive manufacturing processes. SFF does not implement any part-specific tooling. Instead, a three dimensional component is often produced from a graphical representation devised using computer-aided modeling (CAM). This computer representation may be, for example, a layer-by-layer slicing of the component shape into consecutive two dimensional layers, which can then be fed to control equipment to fabricate the part. Alternatively, the manufacturing process may be user controlled instead of computer controlled. Generally speaking, a component may be manufactured using SFF by successively building feedstock layers representing successive cross-sectional component slices. Although there are numerous SFF systems that use different components and feedstock materials to build a component, SFF systems can be broadly described as having an automated platform/positioner for receiving and supporting the feedstock layers during the manufacturing process, a feedstock supplying apparatus that directs the feedstock material to a predetermined region to build the feedstock layers, and an energy source directed toward the predetermined region by a torch. The energy from the energy source modifies the feedstock in a layer-by-layer fashion in the predetermined region to thereby manufacture the component as the successive layers are built onto each other.
One recent implementation of SFF is generally referred to as ion fusion formation (IFF). With IFF, a torch such as a plasma, gas tungsten arc, plasma arc welding, or other torch with an orifice is incorporated in conjunction with a stock feeding mechanism to direct molten feedstock to a targeted surface such as a base substrate or an in-process structure of previously-deposited feedstock. A component is built using IFF by applying small amounts of molten material only where needed in a plurality of deposition steps, resulting in net-shape or near-net-shape parts without the use of patterns, molds, or mandrels. The deposition steps are typically performed in a layer-by-layer fashion wherein slices are taken through a three dimensional electronic model by a computer program. Hence, in most deposition applications it would be considered a layer additive manufacturing process. A positioner then directs the molten feedstock across each layer at a prescribed thickness.
There are also several other SFF process that may be used to manufacture a component. SFF processes can be sub-divided into subcategories such as direct metal deposition (DMD) and selective laser sintering (SLS) to name just a few. DMD is a process whereby metal is melted then placed where needed to build a three-dimensional part. SLS on the other hand spreads a layer of powder on a table then selectively fuses the appropriate portion to build a three-dimensional component. One of the challenges facing SFF processes, and more particularly ion fusion formation (IFF) processes and direct metal deposition (DMD) processes is that of achieving a high deposition accuracy rate, so as to approach a net shape more closely and thereby reduce or eliminate the need for subsequent machining. As machining is reduced, the cost of the component is reduced. However, to be economically viable the deposition rates are preferably high relative to the thickness of the section to be built.
In order to achieve higher deposition accuracy rates, high heat is required. This applies to all IFF and DMD systems but particularly to gaseous systems, such as arc based systems. These types of gaseous systems inherently tend to be more energy diffuse than laser or electron beam systems due to the basic mechanism of heat transfer, and more particularly the impingement of very high temperature gas flow onto a work piece. One inherent limitation of this type of system is the torch gas concentration and the velocity of the gas through an orifice of the torch. To vary the heat flow in a gaseous system one of the variables is the size of the orifice. A large orifice supplies more heat and a smaller orifice less heat, but with greater accuracy. When the orifice size is decreased, the velocity increases, if all other variables remain constant. Gas velocity above a certain level creates splatter. In addition, a small orifice size restricts heat flow due to the gas flow restriction. Consequently, the deposition rate is reduced.
Compounding the issue of high deposition accuracy rates are variations in the thickness of the walls of the component being fabricated. Many components require deposition of areas of high thickness relative to other areas of lesser thickness. Variation of the deposition rate for a gaseous torch from a high rate to a low rate requires variation of the orifice size to match heat transfer to material feed rate. As stated earlier, a higher rate of deposition requires a higher feed rate and more heat. This results in a need for greater gas flow and thus a larger orifice to stay below the velocity that creates spatter.
In addition to SFF, joining of two components using conventional plasma torches nozzles creates relatively large fusion zones compared to other fusion joining processes such as electron beam or laser welding. A narrower orifice could reduce the fusion zone width (diameter) of the plasma weld and possibly increase penetration of the weld. The latter would result from a higher energy density at the plasma spot in the joint.
Current technology requires the orifice for IFF to be changed manually. This is cumbersome as the deposition operation must be stopped, the electric arc turned off and the torch allowed to cool. Additionally, this is very time consuming and inefficient. A preferred approach would be to change the orifice size continuously during operation. Currently, a typical IFF or DMD system has the capacity to change orifice size by removing one orifice and installing another of different size. Ideally a change in the orifice size, and more particularly the diameter of the orifice, would be accomplished without physically replacing the orifice as occurs now. Thus, as a part is built the nozzle would change diameter depending on how much heat was required to build a given feature. As more heat is needed to achieve higher deposition rates the orifice diameter would be enlarged. For more accurate depositions, the orifice diameter would be reduced to reduce the heat spot size, perhaps the heat flow, and melt size of the feedstock.
Hence, there is a need for a torch nozzle for use in high deposition rate accuracy applications, such as in solid free-form applications, that includes a torch nozzle, including an orifice that provides for variations in delivery of heat depending on how much heat is required to build a given component feature.